Manufactured components often have to conform to dimensional and/or geometrical tolerances. These mat be specified by the manufacturer or ultimate user of the component e.g. to ensure that the component behaves correctly in service. Thus it is common to subject components to post-manufacture inspection procedures.
A known technique for determining the dimensional accuracy of components involves driving a computer-controlled coordinate measuring probe or stylus over the surface of he component. Such a probe typically comprises a stiff elongate member (formed e.g. of carbon fibre) having a small hard (typically industrial ruby) sphere at the measuring end. An example of a commercially available probe is the Renishaw SP600™ analogue scanning probe. The probe is fitted to a quill (providing z-direction movement), which in turn is mounted to a carriage assembly (providing x- and y-direction movement), and the component mounted on a measuring table, whereby the movements of both the probe and the component can be controlled via a programmable coordinate measuring machine (CMM).
Sensing means associated with the probe detects when and where the sphere contacts a surface and the CMM translates that information into a spatial coordinate for the surface contact point with the sphere. Thus by making contact with the component at spaced apart points along a programmed guide path, the probe can provide discrete measurements of the relative positions of these points. In this way, shape characteristics (such as profiles or outlines) of the actual component can be built up, and these characteristics can then be compared for conformity with e.g. predetermined tolerance limits.
Conventionally, the operator programs a guide path by a teach-and-learn technique in which he drives the probe under manual control to specific points along the desired guide path, and saves the oppositions of those points in the memory of the CMM. When performing a measurement run, the CMM subsequently drives the probe from point-to-point in an order determined by the operator, taking measurements along the way. In defining the guide path, the operator will also specify approach directions and probe angles to ensure that the probe or quill does not collide with the component. As and when necessary, the operator may also program the quill to change the probe (usually at a probe change station remote from the component) so that e.g. a probe with a longer elongate member is used to reach otherwise-inaccessible parts of the component.
Gas turbine engine components, and particularly critical components such as blades, vanes and discs, are commonly inspected using such techniques. For example, blades usually have complex aerofoil shapes and may need to be characterised at a plurality of different sections. Conventionally an inspection probe operator will separately program the inspection path for each section. With a stand-alone blade a typical inspection path may consist of a simple continuous circumnavigation or the blade.
A problem can arise, however, when the component under inspection has a more complex shape. For example, a gas turbine blisk (bladed disk) comprises a central disk or ring with a plurality of angularly spaced blades or vanes extending radially therefrom. It is often not possible for the probe to follow uninterrupted inspection paths on a blisk because the tight angular packing of the blades forces the operator to access different surface positions from different sides of the blisk. Hence a probe inspection path for a blisk typically consists of discrete sub-paths. At the end of a sub-path, the probe may be moved away from the component and reorientated before reengaging with the component from a different direction to start the next sub-path. The probe length may also be changed between sub-paths.
Thus the amount of time an operator needs to program and fully inspect the disk and all the blades on a blisk using conventional procedures can be significantly greater than the time required to program and inspect a stand-alone blade multiplied by the number of blades. Indeed the amount of time can result in conventional inspection becoming economically unviable.